Fluid animation with varying speed
I know it's a late reply, but I think I can help, if I understand correctly.

You can use keyframes on fluid simulations.  The best way to slow down the simulation is to set keyframes in the "Speed" field. 



You will have to set the keyframes and then rebake the fluid simulation so they can be applied, this can be annoying if you are using a large domain and/or high resolution to the simulation as it can take a long time to bake and rebake till its right, so it's better to bake it with a small domain and low resolution so that you can quickly check to see if the fluid is behaving the way you want, then crank-up the domain size/resolution for your final render.